Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km north of Lake Kivu. Also known as Nyamulagira, it has a volume of 500 cu km, and extensive lava flows blanket 1500 sq km of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with its steep-sided neighbor Nyiragongo to the SW. The 3058-m-high summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.